1. Field of the Invention
The present invention relates to lean premix combustion systems in general, and to the detection of a flashback condition in lean premix fuel nozzles of gas turbine combustion systems in particular.
2. Brief Discussion of the Related Art
Many advanced gas turbine combustion systems use lean premix nozzles in pursuit of lower emissions and higher efficiency. Unfortunately, many of these systems have experienced problems associated with instabilities and flashback. Flashback occurs when the flame normally contained to the combustion zone of the gas turbine combustion system, moves back into the fuel nozzle.
When flashback occurs in the fuel nozzle, the temperatures inside the nozzle rise above the design temperature for the nozzle material causing costly damage. Also, upon occurrence of a severe flashback condition, fragments of the nozzle material, usually metal, tend to pass through the turbine system usually causing severe damage to the turbine blades. This type of failure, regardless of frequency, can be catastrophic in terms of down time, maintenance costs and lost revenue.
In order to prevent such damage, many devices have in the past been used to detect flashback in fuel nozzles. However, all previous devices tend to exhibit undesirable characteristics such as slow response time, and point type or line of site measurements.
For example, thermocouples and bimetallic elements when used as flashback detectors in fuel nozzles, suffer from the disadvantages of providing only localized point measurements and generally slow reaction times (typically 2 to 3 minutes), which can lead to failure of the fuel nozzle before detection. Another disadvantage of these sensors is that, since they only detect heat, they are unable to distinguish between heat generated by the flame of a flashback condition and the heat radiated by the normal combustion process of the gas turbine combustion system.
Similarly, flame rods must be in direct contact with the flame in order to function properly. Unfortunately, it is not always known exactly where in the fuel nozzle of a gas turbine combustion system flashback will occur, and unless the flame rods are in the precise location, flame rods would be useless for detecting the flame occurring during a flashback condition.
Attempts to use radiation type flame sensors as flashback detectors have also been made. For this type of detector, a photocell is used as the actual detector. At least one element of the photocell is coated with a sulfide compound, such as cadmium-sulfide or lead-sulfide, so as to be sensitive to the particular wavelengths of light emitted by a flame occurring during a flashback condition. For instance, the electrical resistance of cadmium-sulfide decrease directly with increasing intensity of light, and like lead-sulfide, will function as a variable resistor. However, when used to detect the presence of a flame, a cadmium-sulfide photocell is useful only for sensing that portion of the flame occurring in the visible light wavelengths. Unfortunately, the cadmium-sulfide photocell will not respond to gas flames, and therefore can only be used to detect the presence of oil flames.
On the other hand, a lead-sulfide photocell provides detection in the infrared wavelength regions. Similar to the cadmium-sulfide photocell, the lead-sulfide photocell can change its resistance inversely to the infrared radiation it is subjected to, and the current flow generated by the lead-sulfide photocell serves as a measure of flame strength. However, the xe2x80x9cshimmering effectxe2x80x9d caused by movement of hot gases between a refractory surface and the lead-sulfide photocell can erroneously deceive the photocell into indicating the presence of a flame, which makes this type of photocell unreliable for use as a flashback detector.
To overcome these problems, a suitable flashback detector must be able to reliably and dependably detect the flame of a flashback condition anywhere inside the fuel nozzle and provide a clear indication that a flashback condition exists.
It is well known that a flame, being the result of a chemical reaction between a fuel and oxygen, liberates a large number of electrons. Because of this ionization, the flame is capable of conducting an electrical current. Moreover, a flame can conduct both direct and alternating current, either of which could be utilized to establish an electrical circuit.
Conduction occurs when ionization takes place. The electrons that are liberated from the burning fuel and oxygen molecules are free to move about, thus constituting the current. In addition to the freed electrons, a negative electrode properly situated in the vicinity of the flame would in the process of repelling the freed electrons, also would tend to lose some of its own electrons provided that there were a sufficient number of positive ions, such as from a positive electrode, in the vicinity to attract them. Accordingly, the number of electrons leaving the negative electrode and entering the positive electrode determines the rate of current flow. It is apparent that the current flow depends on the number of positive ions that get near enough to the negative electrode. If the area of one electrode is made several times larger than the other, and that electrode is negative, it will accommodate a larger number of positive ions. This in turn will increase the flow of electrons to the positive electrode. Accordingly, an electrode immersed in the flame would act as one electrode and the combustion chamber wall act as the other electrode.
Hence, an electrode strategically placed in a fuel nozzle, in the vicinity of a location likely to experience flashback, will detect the existence of the flame associated with the flashback condition by way of this flame ionization process. Then, an electrical signal produced by the sensor detecting the existence of the flame would in turn be relayed to the various controllers associated with the proper operation of the gas turbine combustion system.
Therefore, advantageous use of flame ionization techniques could be employed to detect the flame present during a flashback condition. A sensor employing these techniques would make an ideal flashback sensor. The responsive nature of the sensor will also facilitate measurements of flame flicker in the nozzle during operation, should they occur.
It is an object of the present invention to provide a flashback detector for a lean premix combustion system.
It is an additional object of the invention to provide a flashback detector for a lean premix system capable of detecting a flashback condition in a lean premix fuel nozzle.
It is another object of the present invention to provide a flashback detector capable of providing real-time detection of a flame associated with a flashback condition occurring anywhere along the entire length of the fuel nozzle of a combustion system.
It is a further object of the invention to provide a flashback detector that isolates the combustion region of a gas turbine combustion system from the sensor electrode thereby minimizing false indication of flashback.
It is still another object of the invention to provide a reliable flashback detector for a gas combustion system that uses inexpensive electronics and uncomplicated peripheral hardware.
It is yet another object of the invention to provide a flashback detector for a gas turbine combustion system that can easily incorporate the detector in new fuel nozzle designs as well as in existing fuel nozzle designs.
It is yet a further object of the invention to provide detection of flame in the fuel nozzle of a gas turbine combustion system within a time frame that will prevent damage to any part of the gas turbine combustion system.
These and other objects, aspects and advantages will be better understood from the following detailed description of preferred embodiments of the invention with reference to the appended claims.
Basically, the present invention is a system for detecting a flashback condition in a fuel nozzle of a lean premix combustion apparatus. The system comprises a sensor positioned in the fuel nozzle and a control circuit coupled to the sensor. The sensor includes a first electrode and a second electrode held in a coplanar but spaced apart manner by an insulating body. At least a portion of the first and second electrodes are exposed to gases flowing through the fuel nozzle. The control circuit coupled to the first and second electrodes of the sensor is capable both of causing electrical fields to radiate 360xc2x0 from the electrodes to the walls of the fuel nozzle and the combustion chamber and of receiving from the electrodes an electronic signal indicating the occurrence of a flashback condition in the fuel nozzle. The first and second electrodes can be spaced apart and insulated by a thermoplastic material or by a ceramic material. Preferably, the sensor is centered in the fuel nozzle center body at a location proximate to the combustion chamber of the gas turbine apparatus. Preferably, the fuel nozzle for use with this invention is a lean premix fuel nozzle.
Ideally, the sensor for detecting a flashback condition in a combustion system, comprises first and second electrodes, an electronic circuit, an electrically and thermally insulating body portion having a relatively smooth surface. The first and second electrodes being operably connected to the electronic circuit, and the first and second electrodes being situated on the relatively planar surface of the body portion in a spaced apart and physically isolated relationship one from the other, whereby upon the occurrence of a flashback condition the first and second electrodes will forward to the electronic circuit a signal indicating that the flashback condition exists. Preferably, the sensor is positioned in the fuel nozzle of the gas turbine combustion system. To provide improved performance the sensor is centered near the downstream end of the center body of the fuel nozzle at a location proximate the combustion chamber of the gas turbine combustion system.
The invention also includes a method for signaling a flashback condition in a fuel nozzle of a combustion system using an electronic detector having a sensor electrode and a guard electrode arranged in a coplanar but spaced apart manner on the surface of an insulating detector body, and an electronic detector circuit. The method comprising the steps of; a) locating the detector body on the center body of the fuel nozzle at a location proximate the combustion chamber such that the sensor electrode and the guard electrode are immersed in the fuel/air stream flowing through the fuel nozzle; b)generating electrical fields from each of the sensor electrode and the guard electrode, the electrical fields extending from the face of the sensor electrode to the walls of the fuel nozzle and the electrical fields extending from the face of the guard electrode to the wall of the combustion chamber; and c) monitoring the sensor electrode and the guard electrode with the detector for the completion of an electrical circuit and the occurrence of a flashback condition in the fuel nozzle.
Preferably, the method for detecting a flashback condition in a lean premix fuel nozzle of a gas turbine apparatus using an electronic detector and an electronic detector circuit, the detector having a first electrode and a second electrode arranged in a coplanar but spaced apart manner on the surface of an insulating body. The method comprising the steps of: a) locating the detector on the center body of the fuel nozzle at a location proximate the combustion chamber of the gas turbine apparatus such that the electrodes are immersed in the gaseous stream flowing through the fuel nozzle; b) generating electrical fields from each of the electrodes; and c) monitoring the first and second electrodes with the detector circuit for the completion of an electrical circuit and occurrence of a flashback condition in the fuel nozzle.
The step of generating electrical fields may further comprise causing the electrical field to extend from the face of the first electrode along the entire length of the walls of the fuel nozzle, and causing the electrical field to extend from the face of the second electrode to the wall of the combustion chamber. The first and second electrodes may be so arranged within the nozzle that first electrode is a guard electrode and the second electrode is a sense electrode, and said guard electrode creates a guard electric field region in the combustion chamber at the sensor, such that it prevents normal combustion ionization from producing detectable current at the sense electrode.